One type of emission tomographic system employed in diagnostic medicine is the single photon emission computed tomography (SPECT) system in which a low-level gamma ray emitter is injected into the body of a patient. The gamma ray emitter is conventionally of a type which preferentially travels to an organ whose image is to be produced. A large-area planar gamma ray detector detects gamma rays emitted from the body normal to its plane. This information is digitally stored as an image in an M by N array of elements called pixels. The values of M and N are conventionally equal to each other, and are commonly 64 or 128 units, or pixels, across the two dimensions of the image.
A SPECT system employs a plurality of views each taken by positioning a detector parallel to, and at an angle about a fixed axis. The angle is incremented in equal steps between views The images thus captured are computer-processed to construct pictures of transaxial slices of the body.
In order to minimize the radiation dosage to which the patient is exposed, the injected gamma ray materials are of relatively low radioactivity. As a consequence, each view requires a substantial time such as, for example, about 40 seconds, to produce. If a total of 64 views on a 360-degree arc is desired, angularly spaced apart by about 5.6 degrees, then the entire imaging process takes about 40 minutes to complete. Blurring or distortion can take place when the body being imaged moves a distance on the order of one image pixel. A typical image pixel is about one-half centimeter square. Keeping a human body still to within one-half centimeter for 40 minutes is difficult, if not impossible. Thus, body motion and resultant image degradation are common.
To further complicate the problem, the emission of the gamma rays from the body is not uniform and depends upon the density of the material through which the radiation passes. If uniform attenuation of the material is assumed then incorrect quantitative measurement can result. Two methods have been described to compensate for this attenuation problem caused by varying density in the test subject Both methods use transmission computed tomography (TCT) to compensate for the variations in density.
The first method utilizes a transmission study which is performed prior to the emission study to determine the distribution of attenuation coefficients to apply to the emission data to compensate for attenuation variations. By performing the transmission study prior to the emission study the scan time is doubled and the problem of misregistration caused by patient movement increases.
The second method utilizes simultaneous measurement of transmission information and emission information to determine the distribution of attenuation coefficients. This method utilizes two different sources of radiation which must be discriminated between to properly isolate the transmitted radiation from the emitted radiation. This discrimination is difficult to perform and the emission data is often distorted by the transmitted radiation. Furthermore, the effects of backscattered radiation can further influence the emitted radiation values. Finally, the simultaneous method requires the use of two distinct radioisotopes thus requiring compensation for variations in attenuation based upon energy variations in the radiation.
Accordingly, it is an object of the present invention to provide a method and apparatus to compensate for the attenuation variations in a test subject while minimizing the time to perform such compensation and to compensate for the variations in attenuation in a subject without affecting the emitted radiation measurements.